Serine biosynthesis as a novel therapeutic target for dilated cardiomyopathy.

AIMS: Genetic dilated cardiomyopathy (DCM) is a leading cause of heart failure. Despite significant progress in understanding the genetic aetiologies of DCM, the molecular mechanisms underlying the pathogenesis of familial DCM remain unknown, translating to a lack of disease-specific therapies. The discovery of novel targets for the treatment of DCM was sought using phenotypic sceening assays in induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) that recapitulate the disease phenotypes in vitro. METHODS AND RESULTS: Using patient-specific iPSCs carrying a pathogenic TNNT2 gene mutation (p.R183W) and CRISPR-based genome editing, a faithful DCM model in vitro was developed. An unbiased phenotypic screening in TNNT2 mutant iPSC-derived cardiomyocytes (iPSC-CMs) with small molecule kinase inhibitors (SMKIs) was performed to identify novel therapeutic targets. Two SMKIs, Gö 6976 and SB 203580, were discovered whose combinatorial treatment rescued contractile dysfunction in DCM iPSC-CMs carrying gene mutations of various ontologies (TNNT2, TTN, LMNA, PLN, TPM1, LAMA2). The combinatorial SMKI treatment upregulated the expression of genes that encode serine, glycine, and one-carbon metabolism enzymes and significantly increased the intracellular levels of glucose-derived serine and glycine in DCM iPSC-CMs. Furthermore, the treatment rescued the mitochondrial respiration defects and increased the levels of the tricarboxylic acid cycle metabolites and ATP in DCM iPSC-CMs. Finally, the rescue of the DCM phenotypes was mediated by the activating transcription factor 4 (ATF4) and its downstream effector genes, phosphoglycerate dehydrogenase (PHGDH), which encodes a critical enzyme of the serine biosynthesis pathway, and Tribbles 3 (TRIB3), a pseudokinase with pleiotropic cellular functions. CONCLUSIONS: A phenotypic screening platform using DCM iPSC-CMs was established for therapeutic target discovery. A combination of SMKIs ameliorated contractile and metabolic dysfunction in DCM iPSC-CMs mediated via the ATF4-dependent serine biosynthesis pathway. Together, these findings suggest that modulation of serine biosynthesis signalling may represent a novel genotype-agnostic therapeutic strategy for genetic DCM.

RNA-dependent synthesis of ergosteryl-3ß-O-glycine in Ascomycota expands the diversity of steryl-amino acids.

A wide range of bacteria possess virulence factors such as aminoacyl-tRNA transferases (ATTs) that are capable of rerouting aminoacyl-transfer RNAs away from protein synthesis to conjugate amino acids onto glycerolipids. We recently showed that, although these pathways were thought to be restricted to bacteria, higher fungi also possess ergosteryl-3ß-O-L-aspartate synthases (ErdSs), which transfer the L-Asp moiety of aspartyl-tRNAAsp onto the 3ß-OH group of ergosterol (Erg), yielding ergosteryl-3ß-O-L-aspartate (Erg-Asp). Here, we report the discovery, in fungi, of a second type of fungal sterol-specific ATTs, namely, ergosteryl-3ß-O-glycine (Erg-Gly) synthase (ErgS). ErgS consists of a freestanding DUF2156 domain encoded by a gene distinct from and paralogous to that of ErdS. We show that the enzyme only uses Gly-tRNAGly produced by an independent glycyl-tRNA synthetase (GlyRS) to transfer glycine onto the 3ß-OH of Erg, producing Erg-Gly. Phylogenomics analysis also show that the Erg-Gly synthesis pathway exists only in Ascomycota, including species of biotechnological interest, and more importantly, in human pathogens, such as Aspergillus fumigatus. The discovery of a second type of Erg-aa not only expands the repertoire of this particular class of fungal lipids but suggests that Erg-aa synthases might constitute a genuine subfamily of lipid-modifying ATTs.

Serine, glycine and one­carbon metabolism in cancer (Review).

Serine/glycine biosynthesis and one­carbon metabolism are crucial in sustaining cancer cell survival and rapid proliferation, and of high clinical relevance. Excessive activation of serine/glycine biosynthesis drives tumorigenesis and provides a single carbon unit for one­carbon metabolism. One­carbon metabolism, which is a complex cyclic metabolic network based on the chemical reaction of folate compounds, provides the necessary proteins, nucleic acids, lipids and other biological macromolecules to support tumor growth. Moreover, one­carbon metabolism also maintains the redox homeostasis of the tumor microenvironment and provides substrates for the methylation reaction. The present study reviews the role of key enzymes with tumor­promoting functions and important intermediates that are physiologically relevant to tumorigenesis in serine/glycine/one­carbon metabolism pathways. The related regulatory mechanisms of action of the key enzymes and important intermediates in tumors are also discussed. It is hoped that investigations into these pathways will provide new translational opportunities for human cancer drug development, dietary interventions, and biomarker identification.

Repurposing the Antidepressant Sertraline as SHMT Inhibitor to Suppress Serine/Glycine Synthesis-Addicted Breast Tumor Growth.

Metabolic rewiring is a hallmark of cancer that supports tumor growth, survival, and chemotherapy resistance. Although normal cells often rely on extracellular serine and glycine supply, a significant subset of cancers becomes addicted to intracellular serine/glycine synthesis, offering an attractive drug target. Previously developed inhibitors of serine/glycine synthesis enzymes did not reach clinical trials due to unfavorable pharmacokinetic profiles, implying that further efforts to identify clinically applicable drugs targeting this pathway are required. In this study, we aimed to develop therapies that can rapidly enter the clinical practice by focusing on drug repurposing, as their safety and cost-effectiveness have been optimized before. Using a yeast model system, we repurposed two compounds, sertraline and thimerosal, for their selective toxicity against serine/glycine synthesis-addicted breast cancer and T-cell acute lymphoblastic leukemia cell lines. Isotope tracer metabolomics, computational docking, enzymatic assays, and drug-target interaction studies revealed that sertraline and thimerosal inhibit serine/glycine synthesis enzymes serine hydroxymethyltransferase and phosphoglycerate dehydrogenase, respectively. In addition, we demonstrated that sertraline's antiproliferative activity was further aggravated by mitochondrial inhibitors, such as the antimalarial artemether, by causing G1-S cell-cycle arrest. Most notably, this combination also resulted in serine-selective antitumor activity in breast cancer mouse xenografts. Collectively, this study provides molecular insights into the repurposed mode-of-action of the antidepressant sertraline and allows to delineate a hitherto unidentified group of cancers being particularly sensitive to treatment with sertraline. Furthermore, we highlight the simultaneous inhibition of serine/glycine synthesis and mitochondrial metabolism as a novel treatment strategy for serine/glycine synthesis-addicted cancers.

Malyl-CoA lyase provides glycine/glyoxylate synthesis in type I methanotrophs.

The biochemical routes for assimilation of one-carbon compounds in bacteria require many clarifications. In this study, the role of malyl-CoA lyase in the metabolism of the aerobic type I methanotroph Methylotuvimicrobium alcaliphilum 20Z has been investigated by gene inactivation and biochemical studies. The functionality of the enzyme has been confirmed by heterologous expression in Escherichia coli. The mutant strain lacking Mcl activity demonstrated the phenotype of glycine auxotrophy. The genes encoding malyl-CoA lyase are present in the genomes of all methanotrophs, except for representatives of the phylum Verrucomicrobium. We suppose that malyl-CoA lyase is the enzyme that provides glyoxylate and glycine synthesis in the type I methanotrophs supporting carbon assimilation via the serine cycle in addition to the major ribulose monophosphate cycle.

Serine restriction alters sphingolipid diversity to constrain tumour growth.

Serine, glycine and other nonessential amino acids are critical for tumour progression, and strategies to limit their availability are emerging as potential therapies for cancer1-3. However, the molecular mechanisms driving this response remain unclear and the effects on lipid metabolism are relatively unexplored. Serine palmitoyltransferase (SPT) catalyses the de novo biosynthesis of sphingolipids but also produces noncanonical 1-deoxysphingolipids when using alanine as a substrate4,5. Deoxysphingolipids accumulate in the context of mutations in SPTLC1 or SPTLC26,7-or in conditions of low serine availability8,9-to drive neuropathy, and deoxysphinganine has previously been investigated as an anti-cancer agent10. Here we exploit amino acid metabolism and the promiscuity of SPT to modulate the endogenous synthesis of toxic deoxysphingolipids and slow tumour progression. Anchorage-independent growth reprogrammes a metabolic network involving serine, alanine and pyruvate that drives the endogenous synthesis and accumulation of deoxysphingolipids. Targeting the mitochondrial pyruvate carrier promotes alanine oxidation to mitigate deoxysphingolipid synthesis and improve spheroid growth, similar to phenotypes observed with the direct inhibition of SPT or ceramide synthesis. Restriction of dietary serine and glycine potently induces the accumulation of deoxysphingolipids while decreasing tumour growth in xenograft models in mice. Pharmacological inhibition of SPT rescues xenograft growth in mice fed diets restricted in serine and glycine, and the reduction of circulating serine by inhibition of phosphoglycerate dehydrogenase (PHGDH) leads to the accumulation of deoxysphingolipids and mitigates tumour growth. The promiscuity of SPT therefore links serine and mitochondrial alanine metabolism to membrane lipid diversity, which further sensitizes tumours to metabolic stress.

High level and enantioselective production of L-phenylglycine from racemic mandelic acid by engineered Escherichia coli using response surface methodology.

L-Phenylglycine (L-PHG) is a member of unnatural amino acids, and becoming more and more important as intermediate for pharmaceuticals, food additives and agrochemicals. However, the existing synthetic methods for L-PHG mainly rely on toxic cyanide chemistry and multistep processes. To provide green, safe and high enantioselective alternatives, we envisaged cascade biocatalysis for the one-pot synthesis of L-PHG from racemic mandelic acid. A engineered E. coli strain was established to co-express mandelate racemase, D-mandelate dehydrogenase and L-leucine dehydrogenase and catalyze a 3-step reaction in one pot, enantioselectively transforming racemic mandelic acid to give L-PHG (e.e. >99 %). After the conditions for biosynthesis of L-PHG optimized by response surface methodology, the yield and space-time yield of L-PHG can reach 87.89 % and 79.70 g·L-1·d-1, which was obviously improved. The high-yielding and enantioselective synthetic methods use cheap and green reagents, and E. coli whole-cell catalysts, thus providing green and useful alternative methods for manufacturing L-PHG.

Genetic engineering approaches for the fermentative production of phenylglycines.

L-phenylglycine (L-Phg) is a rare non-proteinogenic amino acid, which only occurs in some natural compounds, such as the streptogramin antibiotics pristinamycin I and virginiamycin S or the bicyclic peptide antibiotic dityromycin. Industrially, more interesting than L-Phg is the enantiomeric D-Phg as it plays an important role in the fine chemical industry, where it is used as a precursor for the production of semisynthetic ß-lactam antibiotics. Based on the natural L-Phg operon from Streptomyces pristinaespiralis and the stereo-inverting aminotransferase gene hpgAT from Pseudomonas putida, an artificial D-Phg operon was constructed. The natural L-Phg operon, as well as the artificial D-Phg operon, was heterologously expressed in different actinomycetal host strains, which led to the successful production of Phg. By rational genetic engineering of the optimal producer strains S. pristinaespiralis and Streptomyces lividans, Phg production could be improved significantly. Here, we report on the development of a synthetic biology-derived D-Phg pathway and the optimization of fermentative Phg production in actinomycetes by genetic engineering approaches. Our data illustrate a promising alternative for the production of Phgs.

Biocatalytic production of D-p-hydroxyphenylglycine by optimizing protein expression and cell wall engineering in Escherichia coli.

D-p-hydroxyphenylglycine (D-HPG) functions as an intermediate and has important value in antibiotic industries. The high pollution and costs from chemical processes make biotechnological route for D-HPG highly desirable. Here, a whole-cell transformation process by D-hydantoinase(Hase) and D-carbamoylase(Case) was developed to produce D-HPG from DL-hydroxyphenylhydantoin(DL-HPH) in Escherichia coli. The artificially designed ribosome binding site with strong intensity significantly facilitated the protein expression of limiting step enzyme Case. Next, the cell wall permeability was improved by disturbing the peptidoglycan structure by overproduction of D,D-carboxypeptidases without obviously affecting cell growth, to increase the bioavailability of low soluble hydantoin substrate. By fine-tuning regulation of expression level of D,D-carboxypeptidase DacB, the final production yield of D-HPG increased to 100% with 140 mM DL-HPH substrate under the optimized transformation conditions. This is the first example to enhance bio-productivity of chemicals by cell wall engineering and creates a new vision on biotransformation of sparingly soluble substrates. Additionally, the newly demonstrated 'hydroxyl occupancy' phenomenon when Case reacts with hydroxyl substrates provides a referential information for the enzyme engineering in future.

De novo synthesis of serine and glycine fuels purine nucleotide biosynthesis in human lung cancer tissues.

Nucleotide synthesis is essential to proliferating cells, but the preferred precursors for de novo biosynthesis are not defined in human cancer tissues. We have employed multiplexed stable isotope-resolved metabolomics to track the metabolism of [13C6]glucose, D2-glycine, [13C2]glycine, and D3-serine into purine nucleotides in freshly resected cancerous and matched noncancerous lung tissues from nonsmall cell lung cancer (NSCLC) patients, and we compared the metabolism with established NSCLC PC9 and A549 cell lines in vitro Surprisingly, [13C6]glucose was the best carbon source for purine synthesis in human NSCLC tissues, in contrast to the noncancerous lung tissues from the same patient, which showed lower mitotic indices and MYC expression. We also observed that D3-Ser was preferentially incorporated into purine rings over D2-glycine in both tissues and cell lines. MYC suppression attenuated [13C6]glucose, D3-serine, and [13C2]glycine incorporation into purines and reduced proliferation in PC9 but not in A549 cells. Using detailed kinetic modeling, we showed that the preferred use of glucose as a carbon source for purine ring synthesis in NSCLC tissues involves cytoplasmic activation/compartmentation of the glucose-to-serine pathway and enhanced reversed one-carbon fluxes that attenuate exogenous serine incorporation into purines. Our findings also indicate that the substrate for de novo nucleotide synthesis differs profoundly between cancer cell lines and fresh human lung cancer tissues; the latter preferred glucose to exogenous serine or glycine but not the former. This distinction in substrate utilization in purine synthesis in human cancer tissues should be considered when targeting one-carbon metabolism for cancer therapy.

mTORC1 amplifies the ATF4-dependent de novo serine-glycine pathway to supply glycine during TGF-ß1-induced collagen biosynthesis.

The differentiation of fibroblasts into a transient population of highly activated, extracellular matrix (ECM)-producing myofibroblasts at sites of tissue injury is critical for normal tissue repair. Excessive myofibroblast accumulation and persistence, often as a result of a failure to undergo apoptosis when tissue repair is complete, lead to pathological fibrosis and are also features of the stromal response in cancer. Myofibroblast differentiation is accompanied by changes in cellular metabolism, including increased glycolysis, to meet the biosynthetic demands of enhanced ECM production. Here, we showed that transforming growth factor-ß1 (TGF-ß1), the key pro-fibrotic cytokine implicated in multiple fibrotic conditions, increased the production of activating transcription factor 4 (ATF4), the transcriptional master regulator of amino acid metabolism, to supply glucose-derived glycine to meet the amino acid requirements associated with enhanced collagen production in response to myofibroblast differentiation. We further delineated the signaling pathways involved and showed that TGF-ß1-induced ATF4 production depended on cooperation between canonical TGF-ß1 signaling through Smad3 and activation of mechanistic target of rapamycin complex 1 (mTORC1) and its downstream target eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). ATF4, in turn, promoted the transcription of genes encoding enzymes of the de novo serine-glycine biosynthetic pathway and glucose transporter 1 (GLUT1). Our findings suggest that targeting the TGF-ß1-mTORC1-ATF4 axis may represent a novel therapeutic strategy for interfering with myofibroblast function in fibrosis and potentially in other conditions, including cancer.

Formylglycine-generating enzymes for site-specific bioconjugation.

Site-specific bioconjugation strategies offer many possibilities for directed protein modifications. Among the various enzyme-based conjugation protocols, formylglycine-generating enzymes allow to posttranslationally introduce the amino acid Cα-formylglycine (FGly) into recombinant proteins, starting from cysteine or serine residues within distinct consensus motifs. The aldehyde-bearing FGly-residue displays orthogonal reactivity to all other natural amino acids and can, therefore, be used for site-specific labeling reactions on protein scaffolds. In this review, the state of research on catalytic mechanisms and consensus motifs of different formylglycine-generating enzymes, as well as labeling strategies and applications of FGly-based bioconjugations are presented.

The Glycine Lipids of Bacteroides thetaiotaomicron Are Important for Fitness during Growth In Vivo and In Vitro.

Acylated amino acids function as important components of the cellular membrane in some bacteria. Biosynthesis is initiated by the N-acylation of the amino acid, and this is followed by subsequent O-acylation of the acylated molecule, resulting in the production of the mature diacylated amino acid lipid. In this study, we use both genetics and liquid chromatography-mass spectrometry (LC-MS) to characterize the biosynthesis and function of a diacylated glycine lipid (GL) species produced in Bacteroides thetaiotaomicron We, and others, have previously reported the identification of a gene, named glsB in this study, that encodes an N-acyltransferase activity responsible for the production of a monoacylated glycine called N-acyl-3-hydroxy-palmitoyl glycine (or commendamide). In all of the Bacteroidales genomes sequenced so far, the glsB gene is located immediately downstream from a gene, named glsA, that is also predicted to encode a protein with acyltransferase activity. We use LC-MS to show that the coexpression of glsB and glsA results in the production of GL in Escherichia coli We constructed a deletion mutant of the glsB gene in B. thetaiotaomicron, and we confirm that glsB is required for the production of GL in B. thetaiotaomicron Moreover, we show that glsB is important for the ability of B. thetaiotaomicron to adapt to stress and colonize the mammalian gut. Therefore, this report describes the genetic requirements for the biosynthesis of GL, a diacylated amino acid species that contributes to fitness in the human gut bacterium B. thetaiotaomicronIMPORTANCE The gut microbiome has an important role in both health and disease of the host. The mammalian gut microbiome is often dominated by bacteria from the Bacteroidales, an order that includes Bacteroides and Prevotella In this study, we have identified an acylated amino acid, called glycine lipid, produced by Bacteroides thetaiotaomicron, a beneficial bacterium originally isolated from the human gut. In addition to identifying the genes required for the production of glycine lipids, we show that glycine lipids have an important role during the adaptation of B. thetaiotaomicron to a number of environmental stresses, including exposure to either bile or air. We also show that glycine lipids are important for the normal colonization of the murine gut by B. thetaiotaomicron This work identifies glycine lipids as an important fitness determinant in B. thetaiotaomicron and therefore increases our understanding of the molecular mechanisms underpinning colonization of the mammalian gut by beneficial bacteria.

Recognition and ER Quality Control of Misfolded Formylglycine-Generating Enzyme by Protein Disulfide Isomerase.

Multiple sulfatase deficiency (MSD) is a fatal, inherited lysosomal storage disorder characterized by reduced activities of all sulfatases in patients. Sulfatases require a unique post-translational modification of an active-site cysteine to formylglycine that is catalyzed by the formylglycine-generating enzyme (FGE). FGE mutations that affect intracellular protein stability determine residual enzyme activity and disease severity in MSD patients. Here, we show that protein disulfide isomerase (PDI) plays a pivotal role in the recognition and quality control of MSD-causing FGE variants. Overexpression of PDI reduces the residual activity of unstable FGE variants, whereas inhibition of PDI function rescues the residual activity of sulfatases in MSD fibroblasts. Mass spectrometric analysis of a PDI+FGE variant covalent complex allowed determination of the molecular signature for FGE recognition by PDI. Our findings highlight the role of PDI as a disease modifier in MSD, which may also be relevant for other ER-associated protein folding pathologies.

EWS-FLI1 reprograms the metabolism of Ewing sarcoma cells via positive regulation of glutamine import and serine-glycine biosynthesis.

Ewing sarcoma (EWS) is a soft tissue and bone tumor that occurs primarily in adolescents and young adults. In most cases of EWS, the chimeric transcription factor, EWS-FLI1 is the primary oncogenic driver. The epigenome of EWS cells reflects EWS-FLI1 binding and activation or repression of transcription. Here, we demonstrate that EWS-FLI1 positively regulates the expression of proteins required for serine-glycine biosynthesis and uptake of the alternative nutrient source glutamine. Specifically, we show that EWS-FLI1 activates expression of PHGDH, PSAT1, PSPH, and SHMT2. Using cell-based studies, we also establish that EWS cells are dependent on glutamine for cell survival and that EWS-FLI1 positively regulates expression of the glutamine transporter, SLC1A5 and two enzymes involved in the one-carbon cycle, MTHFD2 and MTHFD1L. Inhibition of serine-glycine biosynthesis in EWS cells impacts their redox state leading to an accumulation of reactive oxygen species, DNA damage, and apoptosis. Importantly, analysis of EWS primary tumor transcriptome data confirmed that the aforementioned genes we identified as regulated by EWS-FLI1 exhibit increased expression compared with normal tissues. Furthermore, retrospective analysis of an independent data set generated a significant stratification of the overall survival of EWS patients into low- and high-risk groups based on the expression of PHGDH, PSAT1, PSPH, SHMT2, SLC1A5, MTHFD2, and MTHFD1L. In summary, our study demonstrates that EWS-FLI1 reprograms the metabolism of EWS cells and that serine-glycine metabolism or glutamine uptake are potential targetable vulnerabilities in this tumor type.

Characterization of mycosporine-like amino acids in the cyanobacterium Nostoc verrucosum.

The aquatic cyanobacterium Nostoc verrucosum forms macroscopic colonies in streams, and its appearance is superficially similar to that of the terrestrial cyanobacterium Nostoc commune. N. verrucosum is sensitive to desiccation, unlike N. commune, although these Nostoc cyanobacterial species share physiological features, including massive extracellular polysaccharide production and trehalose accumulation capability. In this study, water-soluble sunscreen pigments of mycosporine-like amino acids (MAAs) were characterized in N. verrucosum, and the mysABCD genes responsible for MAA biosynthesis in N. verrucosum and N. commune were compared. N. verrucosum produced porphyra-334 and shinorine, with porphyra-334 accounting for >90% of the total MAAs. Interestingly, porphyra-334 is an atypical cyanobacteial MAA, whereas shinorine is known as a common and dominant MAA in cyanobacteria. Porphyra-334 from N. verrucosum showed little or no radical scavenging activity in vitro, although the glycosylated derivatives of porphyra-334 from N. commune are potent radical scavengers. The presence of the mysABCD gene cluster in N. commune strain KU002 (genotype A) supported its porphyra-334 producing capability via the Nostoc-type mechanism, although the genotype A of N. commune mainly produces the arabinose-bound porphyra-334. The mysABC gene cluster was conserved in N. verrucosum, but the mysD gene was not included in the cluster. These results suggest that the mysABCD gene products are involved in the biosynthesis of porphyra-334 commonly in these Nostoc species, and that the genotype A of N. commune additionally acquired the glycosylation of porphyra-334.

Integrating T7 RNA Polymerase and Its Cognate Transcriptional Units for a Host-Independent and Stable Expression System in Single Plasmid.

Metabolic engineering and synthetic biology usually require universal expression systems for stable and efficient gene expression in various organisms. In this study, a host-independent and stable T7 expression system had been developed by integrating T7 RNA polymerase and its cognate transcriptional units in single plasmid. The expression of T7 RNA polymerase was restricted below its lethal threshold using a T7 RNA polymerase antisense gene cassette, which allowed long periods of cultivation and protein production. In addition, by designing ribosome binding sites, we further tuned the expression capacity of this novel T7 system within a wide range. This host-independent expression system efficiently expressed genes in five different Gram-negative strains and one Gram-positive strain and was also shown to be applicable in a real industrial d- p-hydroxyphenylglycine production system.

Leveraging Formylglycine-Generating Enzyme for Production of Site-Specifically Modified Bioconjugates.

Enzymatic modification of proteins can generate uniquely reactive chemical functionality, enabling site-specific reactions on the protein surface. Formylglycine-generating enzyme (FGE) is one enzyme that can be exploited in this fashion. FGE binds its consensus sequence (CXPXR, known as the "aldehyde-tag") and converts the cysteine to a formylglycine (fGly). fGly-containing proteins contain a bioorthogonal aldehyde on their surface that can be modified selectively in the presence of the 20 canonical amino acids. Here, we describe protocols for the generation of a site-specifically modified protein, an antibody-drug conjugate (ADC), using aldehyde-tagging protocols and aldehyde-reactive conjugation chemistry.

Astrocytes in primary cultures express serine racemase, synthesize d-serine and acquire A1 reactive astrocyte features.

d-Serine is a co-agonist at forebrain N-methyl-d-aspartate receptors (NMDAR) and is synthesized by serine racemase (SR). Although d-serine and SR were originally reported to be localized to glia, recent studies have provided compelling evidence that under healthy physiologic conditions both are localized primarily in neurons. However, in pathologic conditions, reactive astrocytes can also express SR and synthesize d-serine. Since cultured astrocytes exhibit features of reactive astrocytes, we have characterized d-serine synthesis and the expression of enzymes involved in its disposition in primary glial cultures. The levels of SR were quite low early in culture and increased markedly in all astrocytes with the duration in vitro. The concentration of d-serine in the culture medium increased in parallel with SR expression in the astrocytes. Microglia, identified by robust expression of Iba1, did not express SR. While the levels of glial fibrillary acidic protein (GFAP), glycine decarboxylase (GLDC) and phosphoglycerate dehydrogenase (PHGDH), the initial enzyme in the pathway converting glycine to l-serine, remained constant in culture, the expression of lipocalin-2, a marker for pan-reactive astrocytes, increased several-fold. The cultured astrocytes also expressed Complement-3a, a marker for a subpopulation of reactive astrocytes (A1). Astrocytes grown from mice with a copy number variant associated with psychosis, which have four copies of the GLDC gene, showed a more rapid production of d-serine and a reduction in glycine in the culture medium. These results substantiate the conclusion that A1 reactive astrocytes express SR and release d-serine under pathologic conditions, which may contribute to their neurotoxic effects by activating extra-synaptic NMDARs.